Tag: water steam and ice

What is the principle behind adding salt to water to keep the boiling temperature lower? Do other substances have the same effect?

Actually, it’s the other way around! Adding salt or sugar (or anything else that dissolves in water and that doesn’t boil easily itself) to water actually raises the water’s boiling temperature! That’s because the salt or sugar molecules interfere with the evaporation of water molecules and boiling is just a special type of evaporation.

Boiling occurs when the evaporation of water molecules becomes so rapid that bubbles of evaporating water molecules form inside the body of the water itself and are able to grow larger and larger, despite the crushing pressure of the surrounding atmosphere. Below water’s boiling temperature, any bubble of water vapor that forms inside the body of the water will be smashed almost instantly. But at water’s boiling temperature, the pressure of water vapor inside each bubble is high enough to keep the bubble from being crushed. However, adding sugar or salt to the water makes it harder for water molecules to enter one of these water vapor bubbles because the water molecules in the water cling to the salt or sugar molecules and thus don’t evaporate as often. With fewer water molecules entering a water vapor bubble, that bubble can’t sustain itself and is crushed. Only when you heat the salty or sugary water above the boiling temperature of pure water is there enough evaporation into each water vapor bubble to support it against atmospheric pressure.

Why do people put salt on icy sidewalks in the winter?

Whenever a molecule dissolves in water, the water molecules bind to that molecule and surround it, forming a shell of water molecules around the impurity. Salt water is filled with these tiny balls of water, each one surrounding a single salt ion (either a sodium positive ion or a chlorine negative ion). These little water balls can’t crystallize into ice because ice can’t fit a sodium ion or a chlorine ion into its orderly structure. As a result, the presence of salt in the water makes it harder for the water to crystallize into ice. The water has to exclude the salt from the crystals that form as it freezes and this difficult process requires that the salt water be cooled below the freezing temperature of pure water before it will freeze. The more salt the water contains, the lower the temperature at which that salt water will freeze. This effect even works when you just sprinkle salt on ice. As long as the temperature of the ice isn’t too cold, the salt will begin to dissolve in the water molecules of the ice and ice’s crystalline structure will begin to break down. The result will be a puddle of cold salty water. That’s why people use salt to melt the ice on sidewalks. But if the ice is too cold, the salt will remain separate and the ice will stay pure ice. That’s why salting only works when the temperature isn’t too far below freezing.

Is it true that cold water in a pan will boil faster than hot water in a pan?

No. It takes more heat to bring a pan of cold water to boiling than it does to bring an equivalent pan of hot water to boiling. You can see that this must be true by noting that the cold water must first reach the temperature of the hot water, after which both pans will be equivalent. But there are a few interesting peculiarities with freezing and boiling water. One worth noting is that water that has recently been boiled will freeze more easily than water at an equal temperature that hasn’t been boiled. That’s because boiling drives the dissolved gases out of the water so that it can crystallize more easily. The ice that forms from boiled water tends to be unusually clear because it contains very few air bubbles. Water that contains lots of dissolved air traps those air bubbles as it freezes and the air bubbles slow the freezing process.

When you freeze water, are the minerals separated from the molecules of the water? (When I freeze store-bought water and it then thaws out, there is a glob of nasty looking minerals that settle to the bottom of the bottle.)

When water freezes, it forms ice crystals. Crystals are very orderly arrangements of molecules in which each molecule has a particular position and orientation. Each crystal grows from a tiny initial seed crystal by adding one molecule after another to the surfaces of the crystal. Since each molecule that attaches to the crystal must fit into a particular position and have a particular shape and orientation, molecules that are different from those in the crystal tend to be excluded from the growing crystal. Thus an ice crystal that’s growing in dirty water will nonetheless consist almost exclusively of water molecules. Only if the water freezes very quickly will it trap large numbers of impurities by not giving them time to get out of the way. Even gases are excluded from the ice, which is why air bubbles often appear as water freezes into ice.

The minerals that you see in the thawed bottle of water were originally dissolved or suspended in the water. But as the water froze, the ice crystals excluded those impurities and they remained in the liquid portion of the water. Eventually the liquid portion of the water dwindled away and the minerals were forced to come out of solution as solid particles. When the water thawed, those minerals failed to redissolve (they’re often only weakly soluble in water and have great difficulty redissolving).

This phenomenon whereby crystallizing a liquid separates out its impurities is very useful in chemistry—many important chemicals, notably medicines, are purified in this manner. Similarly, freezing water is an important way of purifying it in some locations—native people in cold countries have used sea ice (the pure ice that forms when seawater freezes) as a source of fresh water for centuries. And you may have noticed that when you eat frozen juice, you can suck away the sweet flavored portion and leave behind only the pure ice portion—because the sugar and flavors have been excluded from the pure ice crystals during freezing.

Why is steam so efficient when cooking food and can you explain how so much heat is released when the steam changes phase, i.e. condenses?

Steam is the gaseous form of water and consists of independent water molecules. When steam comes in contact with relatively cool food, the water molecules have the opportunity to stick to one another and the steam condenses into liquid water. While most small molecules bind relatively weakly to one another, water molecules bind remarkably strongly. They form hydrogen bonds, in which negative charge on the oxygen atom of one water molecule attracts positive charge on a hydrogen atom of another water molecule. Water’s hydrogen bonds are so strong that water remains a liquid well above room temperature while most other small molecules (carbon dioxide, methane, nitrogen, oxygen, etc.) are gases even at very cold temperatures. So when water molecules condense from steam to liquid, they form strong bonds with one another and release a great deal of energy. This energy takes the form of heat and it quickly raises the temperature of the food on which the water is condensing. That’s why steam cooks food so quickly and efficiently.

Why does water boil quicker when you add salt?

Salt should actually delay boiling in water by raising the water’s boiling temperature. The water simply won’t boil until its temperature reaches a certain value and that requires heat. There isn’t any trick that will shorten the heating time, except to add chemicals that boil at lower temperatures than water (e.g. alcohol). But sprinkling salt into very hot water may trigger boiling by providing nucleation sites on which the bubbles can begin to form.

Does salt/sugar raise the boiling point or lower it? I thought you added salt to water so that it could form bubbles faster, but if the boiling point goes up isn’t this kind of pointless?

When salt or sugar are dissolved in water, they raise the boiling temperature. It reduces the fraction of water molecules at the surface of the water, so that fewer leave each second. Because of this reduced leaving rate, the water has to get hotter before enough water molecules will leave each second to allow evaporation to occur inside the liquid (i.e. for the water to boil). As for the value of adding salt, sugar, or any other material to water to encourage boiling, that is a very different matter. Adding anything that can serve as a site for bubble formation will help the water to boil. Nucleating the tiny bubbles that eventually grow into the large bubbles we associate with boiling isn’t easy. Often it occurs at a hot spot in the pot, or near a defect on the pot’s inner surface. If there aren’t any hot spots or defects, then adding sharp objects will aid bubble formation. That’s why sprinkling sugar or salt into extremely hot water can help it boil. This boiling occurs before the sugar or salt dissolve. They are just acting as nucleation sites. You’d do just as well to add sand, which doesn’t dissolve at all.

Is water, at say 35° F, more dense than water at 80° F?

Yes, water at 35° F is more dense than water at 80° F, so that the hotter water will float on the colder water. But water is special (and almost unique) in that it does expand slightly as it cools to its freezing temperature. Water’s density reaches a maximum at 3.98° C (about 39° F) and then actually becomes less dense as you cool it toward 0° C (32° F). That means that 33° F water will float on 39° F water! This bizarre behavior allows ice and very cold water to float above slightly warmer water and keeps ponds from freezing solid. Without it, many animals would perish during the winter.

What is the function/purpose of water in your body? What happens if your body doesn’t get sufficient water or gets too much water? What are the best sources of water?

Water plays so many roles in your body that I’ll have to choose which one to describe. The role that strikes me as most important is water’s capacity to dissolve chemicals and carry them about. Our bodies deal with thousands or perhaps millions of different molecules, each of which must migrate from place to place to perform its required tasks. For example, the molecules that are responsible for energy—carbohydrates, fats, oxygen, and carbon dioxide—must all move through us to keep us alive. Water acts as the carrier for all of these molecules. Many of the molecules dissolve easily in water—meaning that water molecules surround these molecules and carry them about one molecule at a time. Carbohydrates, carbon dioxide, salts, and many other organic compounds dissolve easily in water and are carried about by it. Other chemicals, particularly fats and oxygen, don’t dissolve easily in water and need help. Hemoglobin in red blood cells is the molecular structure that carries oxygen through our blood. So my answer to the first question is that water is the principal solvent for all the chemicals in our body.

If your body gets too much or too little water, the dissolved chemicals become either too dilute or too concentrated. When you have too much water in your body, the water tends to flow out of you through any membranes that are permeable to water. Among other things, this process tends to make you excrete extra water as urine. When you drink too much water, it goes right through you. When you have too little water in your body, water tends to flow into you through any permeable membranes, or at least tends not to flow out of you. When you eat lots of salt and are overconcentrated, you don’t excrete much water and feel extremely thirsty—your body is looking for more water.

Finally, the best sources of water are those that simply don’t have many dissolved chemicals; or at least none that cause trouble for your body. That means that your water shouldn’t have much lead or arsenic dissolved in it or any of a number of noxious organic chemicals. The purest waters are distilled water, rain water (assuming minimal air pollution), and water that has been chemically filtered (via ion exchange, reverse osmosis, and/or activated carbon). Spring and well waters tend to contain substantial amounts of dissolved calcium and magnesium salts, which make the water less pure but probably don’t affect its healthfulness. One special case to look out for is water that was very hard but that has been passed through a water softener. The dissolved minerals that made the water hard will have been replaced by sodium compounds during the softening process and excessive sodium consumption may be a problem for some people.